1. Field of the Invention
This invention relates to an assembly for control of the position of the cam ring of hydraulic pumps and motors and in particular for hydraulic pumps having system pressure actuated control pistons for regulating cam ring eccentricity and for pumps wherein the cam ring eccentricity is determined by the action of mechanical springs and other biasing means.
2. Description of the Prior Art
Variable volume vane pumps typically have radially directed, diametrically opposed, pressure actuated pistons for regulating the position of a movable cam ring in response to pump operating pressure conditions. Pumps of this type also regulate cam ring positioning by the action of a single, radial, spring-loaded piston or by a single pressure actuated piston, either of which similarly oppose pressure related cam ring forces. A net unbalanced force is applied radially to the cam ring, a result of the asymmetry of the pump pressure distribution within the pump chamber which is bonded circumferentially by the cam ring. This force is balanced on the ring by the bearing reaction produced by a thrust screw or thrust block assembly that applies an oppositely directed radial reaction to the outer periphery of the ring.
Various methods have been employed to restrain the cam ring with a view to providing the kind and sufficiency of cam ring motion as is required for efficient pump operation.
The requisite ring motion is cyclic in nature. Pump pressure continually produces the radial thrust forces referred to above, and, in one known method, develops frictional forces at the point where the control pistons bear on the movable cam ring. These friction-related forces are known to cause piston wear as well as excessive wear at the pump casing bore into which the pistons fit.
In addition, accurate pump control is considerably hampered by friction on the pistons because ring motion, then, is not smooth and continuous in response to control piston input. It is, instead, random and incremental as static friction is first developed, then overcome to allow ring motion, and later becomes static again. This phenomenon, referred to as slip-stick friction is responsible for the retarded effect of control piston action and results in control less positive and predictable than if friction were absent.
The chafing at the pump case bore eventually produces clearances between the control and bias pistons and the bore. The pistons are exposed to high pressure levels and since leakage varies as the cube of the clearance, the pump become exponentially less efficient as wear clearance accrues. This problem is particularly troublesome in vane pumps where the fretting action between cam ring and control pistons is a result of high cyclic loads applied at high frequency.
The following U.S. patents bearing on the subject matter of this invention are known: U.S. Pat. Nos. 3,052,189; 3,523,746; 2,600,633; 3,137,235, 3,918,855 and 3,901,628.
These patents disclose constant and variable volume vane pumps having several means for control of cam ring position. Essentially three distinct methods are shown to provide a reaction to the radial thrust force which fluid pressure produces on the cam ring: a thrust screw; a pivoting or articulating ball joint and a roller or needle bearing support. In each known method of thrust block reaction, an inner member is maintained in contact with the movable cam ring on its outer periphery as the ring moves in response to pump operating conditions.
The thrust screw approach permits the cam ring to roll on the flat inner surface of the screw as piston forces produce motion which is eccentric of the rotor center. This rolling causes relative motion between the control pistons, which are stationary in the circumferential sense, and the cam ring.
A ball joint may replace the static reaction of the thrust screw. In this technique two blocks are provided; an inner block which maintains cam ring contact and an outer block fixed to the pump casing. A ball bearing is positioned in a suitable surface for its retention between the blocks and permits the inner block to pivot about the bearing axis as the cam ring, moving eccentrically in response to system pressure effects, rotates. There is, again, considerable relative motion which produces friction at the axially loaded control piston--cam ring interface.
A third type of thrust reaction is provided by using a similar arrangement of inner and outer blocks, but with a series of roller or needle bearings in the space provided for their retention. The inner block continually maintains cam ring contact and moves by translation as the ring eccentricity varies in response to control piston forces. Here, the needle bearings preclude rotation and allow sliding action only between the blocks. Thrust reaction magnitudes are generally so large and are applied to bearing surfaces, viz, the needle--block interfaces, which are so small, that excessive contact stresses produce operating lives that are unacceptably short.